1. Field of the Invention
The present invention relates to a calibration apparatus and a calibration method in which an image acquisition apparatus is calibrated, a program for calibration for allowing a computer to function as the calibration apparatus, and a calibration jig to be combined with the calibration apparatus.
2. Description of the Related Art
A calibration method of an image acquisition apparatus, or camera calibration is an important technique in image processing. This technique is used especially in three-dimensional measurement or object recognition using a camera and object grasping by a robot. By a camera calibration technique, optical and geometrical characteristics of the image acquisition apparatus, such as cameras, are specified as described later, and the technique is important also as a problem that projective transformation parameters of the camera, a position and orientation parameter of the camera in an environment, and the like are estimated. To calculate or estimate these camera calibration parameters, a marker (marker or landmark for the calibration) having a known three-dimensional position in a world coordinate frame in which the camera is disposed is photographed by the camera. A two-dimensional position of the image of the marker photographed in the camera image is calculated in the image. Various camera parameters are calculated from a positional relation between the three-dimensional position of the marker and the two-dimensional position of the image of the marker. Various methods of the camera calibration have been known.
One of the methods of the camera calibration is described in J. Weng, P. Cohen, and M. Herniou, “Camera Calibration with Distortion Models and Accuracy Evaluation,” IEEE Transactions of Pattern Analysis and Machine Intelligence, Vol. 14, No. 10, 1992, pp. 965 to 980. In the method described in this document, a plane board on which a plurality of square calibration markers having the same size are arranged, and a z-stage on which the plane board is slid in a z-axis direction are prepared as calibration jigs. Vertices of these squares are recognized by image processing to measure the position or each marker in the image. A z value obtained from a z stage is combined with an xy coordinate on the plane board to measure the three-dimensional position in the world coordinate frame of each marker. Moreover, for of the corresponding marker, the three-dimensional position in the world coordinate frame and the two-dimensional position in a plurality of images are used to estimate the calibration parameters of the image acquisition apparatus.
In B. Girod, et al., edited, Principles of 3D image analysis and synthesis, Kluwer Academic Publishers, 2000, pp. 29 to 30, a method is disclosed in which the calibration markers are mutually arranged on three vertical planes, and the marker is recognized to calibrate the image acquisition apparatus (camera).
In Oliver Faugeras, Three-Dimensional Computer Vision, MIT Press, 1933, pp. 230 to 235, a calibration jig constituted of two planes (calibration boards) is proposed. A plurality of square markers are arranged on each calibration board, and four vertices of each square are extracted in the image processing to estimate the position of the marker in the image. Color of these squares constituting the markers is different from that of background, and therefore a boundary via which two planes can be separated is clear.